EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: m3vuv on March 09, 2022, 08:52:08 pm
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hi all,ive been looking at this schematic to build a psu as i have used it in the past,just thinking with the current pot fully one way it looks like there is no current limit should i insert a small resistor in the path of the pot and pin 2 of the 723?,also if i used a -ve supply like the hyland design with a diode and zenner from one leg of the brige rectifier could i make it go to 0v? TIA.
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If you don't want the current limit defeated when the pot wiper is at the "top" of the symbol in the diagram, then you should insert a suitable resistor between the "top" of the pot R11 and pin 2 of the 723.
In that case, there will always be some fraction of the voltage across the current sense resistor (R5 and R6, not including R1 to R4) fed back to the current limit circuit inside the 723.
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First change I would do to that schematic would be to use a transformer with at least two secondary windings, and use a relay or something to change between winding lengths depending on your output voltage.
For example, have 2 x 15-18v AC secondary windings - if you want 0...12v , connect just one secondary winding, if you want 12...24 / 30v enable the relay to connect the second winding.
This way you'd drop less on the transistors and dissipate less, produce less heat etc.
Could also take it the next step with two independent secondary windings - parallel them for half voltage , double current or series them for full voltage, regular current value.
Toroidal transformers with 12v / 15v/ 18v AC secondary windings are more common and cheap.
Also, no 1000uF capacitors on input, go for 3300-5600uF 63v capacitors, something like that. Whatever's sweetspot capacitance vs price. Can go with 85v rated capacitors as you're dealing with 100-120 Hz DC, not high frequency/very low esr switching power supply requirements.
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what about pulling the 723 ground rail negative in relation to ground will that make it go to zero volts?,i know about swithcing taps etc,i brought a scrap farnell 24v/10a psu from ebay with the transfo missing,i have a transfo from an old ps-30 psu so want to use that,i just need build the control board,its in a nice case with 6x 3055 tranys on decent heatsinks ,all in a nice enclosure.https://www.ebay.co.uk/itm/265444680961 (https://www.ebay.co.uk/itm/265444680961)
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would i be better off running this 723 controler from a separate transfo winding,as my transfo has a auxilary lower ampage winding,also any ideas of the value of what resistor i should use in the top leh of the current pot,im thinking this could do 25a max intermitent,well at least the 6x 5055's and the transfo?.TIA.
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the psu filter cap i have is a spague 22.000uf cap 50v rated.
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I also have an infineon sg3532j ic,would i be better to use that as it has a lower current sense voltage.
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You may be able to find the schematic if you know the exact model ... farnell rebrands a lot of power supplies and that one seems like an old model.
Often these linear power supplies have the big 1-2 or more windings with high current, and also maybe a couple smaller windings (ex 10v - 0 - 10v ) to have +/- 9...15v DC for opamps or comparators. SOME may have another winding, like 8-12v ac, used just to produce 5v for logic, maybe led segment display if any etc, some just use the winding for the positive voltage for opamps/comparators etc
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The original supply wasnt adjustable,i am rebuilding it as more of a lab psu,it was missing the transfo,i have a transfo from an old palstar ps-30 psu,it has the main winding high power and another lover power winding,no center taps tho,maybe that gives a clearer picture,need advice on getting it to go to zero and what value resistor to use in the current adjust pot top leg,i dont need telling what i already know,thanks for the advice tho,but pleased stick to what i have asked not what i already know ok.
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Main issue here is that at low voltage (let's call it zero for the sake of making my weekend mental math easy) and high current (let's say 10 amps), you're dissipating a lot of power - 300 W to be exact - in those series pass transistors. That's tough for the transistors, but it also takes a really big heatsink, which will be quite costly.
Transformer tap switching is by far the easiest way to do this. AnTek has some pretty inexpensive toroidal power transformers that have a pair of secondary windings plus some auxiliary windings that can be used for control power.
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I am fully aware of the transformer tap issue,thing is tho my transfo hasnt taps,i need to work with what i have,there are 6x 2n3055 rca pass transistors on huge heatsinks.its the -ve supply im asking about ok.
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so folks what resistor should i add on the top leg of the pot to give some short circuit protection?,also no definitive answer on pulling the ic ground rail negative,will that make it adjustable down to 0v?? TIA.,seems as per normal i get lots of answers but not to what the original questions were!!.
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you should do some google search, there's a lot of very good schematics on the web, and ways to have a zero volt output voltage with the lm 723
you have mastech psus schematics floating around with all you need ....
the first schematic is very crude and lack many options or protections
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So how about giving me a strait answer?,not a list of things to do,dont you think i have looked on google?,why do you imagine i am asking the questions here !!!?
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The voltage feedback resistors being unusually low values can be made advantage of. Put a 4.7K resistor from between the junction of the divider resistors to pin 4. Then a resistor from the Ref pin 6 to pin 4 to make it about 1.5V when the PSU's output is 0V.
More: The output will be able to come down to about 40mV.
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So how about giving me a strait answer?,not a list of things to do,dont you think i have looked on google?,why do you imagine i am asking the questions here !!!?
Let me rephrase this:
Thank you for the information however it does not address my original question. Could someone please inform me as to the value of resistor I should add to prevent a short circuit?
Once again m3vuv, nobody here is required to answer your question, you have to ask and hope someone is kind enough to spend their time to respond to you, even if it's not exactly the answer you were looking for. Scolding those posting is not how you get an answer, it's how you drive people away and anger them.
As for the "non-answers", remember the saying: Give a man a fish, feed him for a day... teach a man to fish, feed him for life.
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it was not scolding just beeing to the point,dont mean to sound rude just need the question answering not a diatribe ok.
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gnif could write you should ease you written word tone if i may say, it's impolite and rude
IF you did search google should have found simple and efficient psu schematics, even doing an proper eevblog search would have given you the ''strait answers'' you needed, i did post some schematics and better simple designs like the Velleman k7200 psu
or this design with an switched xformer tap if i recall and the attached file with explanations and schematics
https://www.eevblog.com/forum/projects/0-30v-0-10a-power-supply-projects/ (https://www.eevblog.com/forum/projects/0-30v-0-10a-power-supply-projects/)
Here at eevblog we can offer help but we are not obliged in any way to do all your design ??
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Give a man a fish and you may feed him for a day, give the man a fishing rod and you will be saving him from stomach perforation in a moment... >:D
(This is not the beginners section, I can be mean here :P)
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Give a man a fish and you may feed him for a day, give the man a fishing rod and you will be saving him from stomach perforation in a moment... >:D
(This is not the beginners section, I can be mean here :P)
tell that to the russians.
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Given it seems you're searching for something really simple, this might get it done.
The led and the weak resistor adds a ~1.8V offset. Resistors needed some tweaking to optimize adjustment.
Yes, adding a resistor in series with the potentiometer in 2-3 will add a max. current limit. (Let's call it Rmax)
R1-R4 are just to distribute the current between the transistors, so one doesn't take a lot more current than others.
The current is measured by Rlim, in this case R5-R6 in parallel (In this case 0.235Ohms).
Current limit sensing voltage threshold is 0.65V (betwen pins 2-3).
The formulas would be something like this (I might be wrong):
Imin = 0.65V/Rlim = 0.65V/0.235R = 2.76A
Imax = (0.65/Rmax)*(Rmax+R11)/Rlim = (0.65V/220)*(220+500)/0.235 = 9A
This is a very basic way to limit the current, as minimum current is pretty high and the limiting resistor will waster power and a considerable voltage.
As they said, there're lots of LM723 circuits over internet. One using operational amplifiers will do a much better job here.
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The ciruit has many problems - many were alread noted in other threads. Consider the circuit more like a crude idea it is far from complete / finished.
One can limit the adjustment range for the current limit pot like shown with the resistor on top. The adjustment range is still rather small, like 2 A to 6 A.
The LM723 internal current limit is just not well suited for an adjustable current limit. The somewhat better way would be to thake the VBE voltage as the upper limit for the current setting and than subtract from there. The way more expensive SG3532 may be better in this respect, but hard to get a relacement if you need one. Kind of replicating that internal circuit may be an option to slightly improve on the current regulation.
For a real lab supply I would forget about the 723 and look for a different plan / maybe a cheap kit from China.
There are ways with a negative supply to get adjustment all the way to zero voltage, but they are usually not good and would reduce the maximum voltage. There is an easier way to get adjustment to zero (or at least close to zero, like 50 mV): add a resistor from the 7 V reference to the feedback divider, so that zero output voltage corresponds some 1-1.5 V at the feedback input.
The voltage range is limited as there is quite some voltage lost: a dralington ouput stage (~1.5 V), current sharing resistors (too small in the circuit - should be more like 0.22 or 0.33 ohms for 3055 with some 20-30 V), the current limit circuit shunt (up to ~ 1.5 V) and the loss in the 723 ( ~ 2 V ?).
One could get more output voltage or use a lower voltage transformer when the 723 itself is powered via a separate filter capacitor.
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it was not scolding just beeing to the point,dont mean to sound rude just need the question answering not a diatribe ok.
No, it’s you AGAIN being rude and trying to dictate to others how they are supposed to reply to you. Doesn’t work that way, buddy.
I twice linked a page on how to ask questions properly. If you followed it, you wouldn’t be running into the problem of people telling you to do things you’ve already done.
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Are you going to bumb this stupid thread every weak? ::)